Sorry the chatting is so difficult. ;-) I try to avoid being too hard to understand, but I also like giving you a bit of a challenge. You learn more that way, right?

And now, 指摘 オン パレード

むしかしいでした I think this might be, strictly speaking, correct... Just a sec... no, actually, it seems to be a common mistake (see the table for i-adjectives and the note at the end). It should be 難しかったです, which is the form I was going to suggest without knowing whether it was required or not. Damn, I'm getting rusty.

かれは「ブログにもっとポストする」といいました。 The way you have it formed here, it sounds like I'm going to post more, because the quoted phrase is a declaration. What you want to do is put the actual request in the quotes (usually in informal form): 「ブログにもっとポストして（下さい）」といいました。 Same thing goes for questions or whatever: The full form of the sentence can be quoted.

First of all, おもしろい is an i-adjective, so you don't use だ when ending an informal sentence with it (you do for na-adjectives like しずか（な） "quiet"). So, grammatically, おもしろかったと思います would be correct.

However, ～と思います expresses an opinion that some piece of information might be true (or sometimes that you are planning to do something), while おもしろかった, in this case, is your opinion already. So using と思います in this case sounds a little strange. おもしろかったです might be a bit better (although is also sounds a lot like the way that elementary school students end most of their essays ;-)).